1. Field of the Invention
This invention relates to a fixing member used to fix toner images in image-forming apparatus of an electrophotographic system, and a fixing assembly making use of the fixing member.
2. Related Background Art
As a problem having become more important in recent years in image-forming apparatus of an electrophotographic system such as copying machines, printers and those used in facsimile machines, there may be given how demands for what is called energy saving be coped with. Conventional toner image fixing assemblies used in such apparatus are of a system in which toner is melted and then fixed to paper, and hence they consume electric power directly as heat energy. Such electric power occupies a great part of the electric power consumed in the whole main body of the apparatus.
To cope with this, as one directionality toward the reduction of power consumption of fixing assemblies, it is attempted to make on-demand operation (real-time operation in which usually no electric current is flowed to a heater and the heater is turned on only at the time of, e.g., image formation). Stated specifically, a fixing member is made in the form of a thin-wall roller, a thin belt or a film so that the heat capacity can greatly be made smaller than that of conventional thick-wall heating rollers to enable quick rise of the assembly and at the same time cut down power consumption during that time. Making such on-demand operation enables the printing to be quickly started upon receipt of printing signals almost without any wait time. Hence, it is unnecessary to keep the fixing member warm at a high temperature around the printing temperature even at the time of non-printing as done in the case when conventional heating rollers are used, and it can be enough only to keep at normal temperature or at most an appropriate low temperature. The dissipation of heat at the time of keeping the fixing member warm increases in proportion to the difference in temperature between environmental temperature and fixing member""s preset temperature. Because of such heat dissipation, the electric power is always consumed even in the state of non-printing. Accordingly, in a variable situation where the printing is intermittently repeated, making the fixing assembly on-demand operable can bring about a very great effect of power saving.
The on-demand operation can also contribute to energy saving on the following points, too. The fixing of toner images is chiefly performed by heating them with a heater from the inside of a heat fixing member which is commonly of a roller type. In a system making use of a halogen lamp as a heat source and a fixing roller as the fixing member, the glass surface temperature of the halogen lamp reaches a high temperature of 400xc2x0 C. or more. Actual fixing temperature, however, is regulated at about 180xc2x0 C. by detecting the surface temperature of the fixing roller. On the other hand, in an on-demand system making use of a ceramic heater and a fixing film, the same fixing performance as that in the above system making use of a fixing roller is achievable in the state the heater temperature is set to approximately from 190xc2x0 C. to 200xc2x0 C. This means that the latter system, in which the top temperature of the heater can be lowered to about fixing temperature, causes less heat dissipation loss and enables more efficient heating, than the former system. In addition, a surface heating system in which the fixing member is heated on its side coming into contact with paper or the like is advantageous in respect of heat response and also in view of heat efficiency, over an internal heating system in which, as fixing speed is made higher, the fixing member has a higher temperature on the inside and the roller has a lower surface temperature as in the case of conventional roller fixing.
As a system in which the fixing member itself effects self-generation of heat, a system is also known which makes use of, e.g., a self-heating element (such as a resistance heating element) or a magnetic-induction heater. According to such a system, the top temperature can be lowered, and it is a highly efficient system. Also, according to this system, the fixing member can directly be heated at the part vicinal to its surface, and hence the heat response is improved and, compared with the roller fixing system operated by usual halogen lamp heating, stable temperature control can be made with small ripples, making it possible to take latitude effectively between a high-temperature offset region and a low-temperature offset region. Then, as the result, stable images can be formed which are free of any uneven gloss and in a high fixing performance.
In what is called a surf system, which makes use of a film as the fixing member, it is difficult to fix color toner images in a good state as explained below, and, because of use of a thin-layer film, it is difficult to achieve uniform fixing performance especially in forming color images having toners in a large quantity on a transfer medium. There have been such problems. In the fixing of color toner images, toner images may melt in different ways in accordance with any unevenness of paper surface when the images are formed on plain paper. This may cause uneven gloss in the surface of fixed images, or, where a film for OHP (overhead projector) is used as the transfer medium, tends to make the transmittance of OHP images different in accordance with any differences in toner image layer thickness, and it has been difficult to obtain high-grade images.
Accordingly, in order to smooth the surface of fixed images, it is also possible to provide the fixing film with an elastic layer at minimum, like that in conventional roller fixing. For example, the film may be provided with an elastic layer of 100 to 300 xcexcm thick, formed of silicone rubber or the like. However, in conventional surf fixing, the fixing film is brought into contact with a heat-generating source such as a ceramic heater on the back of the film to heat the film by contact conduction over a nip distance of 4 to 6 mm at most. Hence, providing the film with an elastic layer having the above thickness has caused a problem that the film has a slow heat response. This problem may more seriously occur in the fixing of color toner images formed of toners with different colors, superimposed mutually in a plurality of layers, because a large quantity of heat flow is required therefor. Where the film is provided thereon with the above elastic layer, the heat flow coming from the heater can not sufficiently come through the film surface during the time an image-fixing medium such as paper passes through the nip, so that only the toner surface layer may be melted to tend to cause faulty fixing.
In such a case, the fixing performance can be improved by making the heater temperature higher. However, the film, the elastic-layer inner face and the heater holder are exposed to high temperature to undergo damage such as thermal deformation, resulting in a shorter lifetime.
As a countermeasure therefor, one may contemplate incorporating a filler in the elastic layer in order to improve heat conduction of the elastic layer formed of silicone rubber or the like. However, the rubber may increase in hardness to decrease in elasticity, so that the effect attributable to the providing of the film with the elastic layer may be damaged. For the reasons as stated above, it is actually not easy to construct the surf system especially for color-image fixing assemblies.
Accordingly, an object of the present invention is to provide a fixing assembly having been made on-demand operable which can heat a fixing member in a good efficiency, and also can perform good surface heating in a good heat response and can well fix color images which require a high heat utilization efficiency, even though a radiation source is disposed on the internal side of a heat fixing member.
Another object of the present invention is to provide a fixing assembly which can form good color images free of any uneven gloss and in a stable fixing performance, can make the assembly compact and may cause less heat dissipation loss, promising superior economical advantages.
The above objects can be achieved by the invention described below. That is, the present invention is a fixing member used to fix toner images, having the form of a film or belt which is multi-layer constructed to have at least a base layer, an intermediate layer and a surface layer; the base layer at least being formed of a material having radiation-transmitting properties, capable of transmitting radiation coming from a radiation source disposed on the back side of the base layer in non-contact with the fixing member, and the surface layer or intermediate layer being formed of a material having radiation-absorbing properties.
With the above construction, the radiant light coming from a radiant-light source serving as a heat source provided on the back can reach the vicinity of the film or belt surface, and the toner images transported to the part of the film or belt surface can effectively be heated.